Options in developing stemborer-resistant maize: CIMMYT's approaches and experiences

Various insect pests, of which stemborers are the most widely distributed and damaging, affect about 30 out of the 35 million hectares planted with maize in developing countries. Chemical control, biological control; cultural methods and host plant resistance constitute the four general approaches to stemborer control. The use of stemborer-resistant maize increases farming efficiency by both reducing yield losses from stemborer damage and reducing or eliminating the cost of insecticides and other inputs. In the past, CIMMYT followed conventional breeding methods to develop germplasm resistant to stemborers and molecular technology, quantitative trait loci (QTL) in marker-assisted selection (MAS) to select for improved stemborer resistance in elite lines. More recently, CIMMYT has developed the capacity to produce transgenic maize with resistance factor(s) derived from genes that encode delta-endotoxins derived from the soil bacterium Bacillus thuringiensis (Bt). A sub-tropical source population with multiple borer resistance (MBR population) was developed by recombination and recurrent selection under infestation with four stemborer species. Marker-assisted selection is being used in two African countries to promote the transfer of resistance into elite and adapted germplasm. The Insect Resistant Maize for Africa (IRMA) project is pursuing the transfer of Bt-based resistance to adapted maize germplasm, initially in Kenya, but later to other interested African countries. CIMMYT's varietal release strategy is to pyramid Bt genes into maize populations with existing multigenic pest resistance, in order to enhance both the levels and durability of plant resistance to maize pests. This paper discusses the various approaches used at CIMMYT to develop stemborer-resistant maize germplasm. Plusieurs espèces d'insectes infestent environ 30 sur 35 million d'hectares plantés de maïs dans les pays en voie de développement. Parmi ceux-ci, les foreurs sont les plus destructifs et les plus largement repartis. La lutte chimique, biologique, les méthodes culturales et l'utilisation des plantes résistantes forment les quatres approches courantes de lutte contre les foreurs. L'utilisation du maïs résistant aux foreurs augmentent l'efficacité culturale en réduisant les pertes en récoltes dues aux foreurs grâce à la réduction ou l'élimination des dépenses en insecticides ou autres entrants agricoles. Dans le passé CIMMYT a suivi la méthode conventionnelle des multiplications po